The optical properties of copper in zinc oxide

Abstract

The behaviour of Cu in ZnO contrasts strongly with that of Cu in other II-VI compound semiconductors. Internal relaxation between the crystal-field-split states of Cu²⁺, 3d⁹, is essentially non-radiative in ZnO. However, a strong near-infrared luminescence is consistently observed in Cu-doped ZnO, some 0.14 eV above that which would correspond to the absorption seen for Cu²⁺. The form of the excitation spectrum of the green luminescence associated with a charge transfer state contrasts strongly with that of the Cu near-infrared luminescence, and contains three no-phonon lines alpha , beta and gamma previously reported by Broser and co-workers (1978). Detailed magneto-optical properties of these lines are compared with this previous work and lead to the first satisfying attribution of them given in another paper (see ibid., vol.14, p.2859, 1981). Finally, it is noted that a 14.4 meV splitting in the infrared optical absorption probably does not arise from an unexpectedly small spin-orbit splitting in the ²T₂ ground state of Cu_Zn²⁺, but may be due to phonon coupling.